1. Field of the Invention
The present invention relates to a heat exchanger.
2. Description of Related Art
JP-A-2007-225190 discloses a heat exchanger to cool exhaust gas by using cooling water of an engine. Exhaust gas is discharged out of the engine, and a part of the exhaust gas is recirculated to an intake side of the engine by an exhaust gas recirculating device (EGR).
The heat exchanger includes plural flat heat-transmitting tubes, and an outer case having a rectangular cross-section in which the flat tubes are layered. Exhaust gas is introduced into the tubes through an inlet part located at a longitudinal end of the outer case, and exhaust gas is discharged out of the tubes through an outlet part located at the other longitudinal end of the outer case. A main part of the outer case is defined between the inlet part and the outlet part.
Each longitudinal end of the heat-transmitting tube has an enlarged part, and the enlarged parts are bonded to each other when the heat-transmitting tubes are layered. An outer periphery of the bonded enlarged parts is bonded to an inner end wall of the main part of the outer case.
The main part of the outer case has an inlet tube through which cooling water flows into the main part, and an outlet tube through which cooling water flows out of the main part.
Cooling water flows into the main part of the outer case through the inlet tube, and passes outside of the heat-transmitting tubes so as to flow out of the main part of the outer case through the outlet tube.
Exhaust gas is distributed into the heat-transmitting tubes after flowing through the inlet part, and the distributed exhaust gas are collected by the outlet part so as to be discharged after passing through the heat-transmitting tubes. At this time, the exhaust gas passing through the tubes is cooled by the cooling water passing outside of the tubes.
However, when exhaust gas having a temperature of 700-800° C. is cooled by cooling water having a temperature of 90-100° C., cooling water may be locally boiled by exhaust gas adjacent to the inlet part.